1. Field of the Invention
The present invention is related to the use of novel derivatives of pyrazolonaphthyridines as modulators of GABAA α5 for the intended use of therapy for enhancing cognition.
2. Description of the Related Art
The inhibitory neurotransmitter γ-aminobutyric acid (GABA), serves as a ligand for two distinct classes of receptors, GABAA and GABAB. The GABAA class is a ligand-gated ion channel while GABAB is a canonical seven transmembrane G-protein coupled receptor. The GABAA receptor is comprised of a number of subunits, including α, β, γ, and δ. Cloning of the individual subunits of the GABAA receptor has confirmed the existence, so far, of six α subunits, three β subunits, three γ subunits, and one δ subunit. The overall structure of the receptor is a pentamer with a minimum subunit requirement of at least one α subunit, one β subunit, and one γ subunit.
Due to afore mentioned diversity of subunits, there are more than 10,000 possible combinations of the subunits that comprise the GABAA receptor, though not all appear in nature. Specific combinations that have been identified to have biological relevance (and their relative abundance in rat brains, include α1β2γ2 (43%), α2β2/3γ2 (18%), α3βγ2/3 (17%), α2γ1 (8%), α5β3γ2/3 (4%), α6βγ2 (2%), α6β8 (2%), and α4β8 (3%) (Barnard, E. A., et al. (1998) Pharmacol. Rev. 50: 291-313 incorporated herein in its entirety).
There are a number of distinct, small molecule binding sites on the GABAA receptor that modulate the activity of the receptor including sites for benzodiazepines, steroids, barbiturates, ethanol, and convulsants (e.g. picrotoxin). The GABA binding site is located at the α/β interface. A tremendous amount of pharmaceutical research has been invested in identifying compounds that bind to the benzodiazepine binding site (BZ-site), which is located at the α/γ interface. Binding of GABA is greatly modulated by binding of drugs to the BZ-site, which can cause a number of different pharmacological responses. Drugs such as diazepam and zolpidem, agonists of GABAA function, have shown historic success as anxiolytic agents (Muller, W. E. (1988) Drugs of Today 24: 649-663 incorporated herein in its entirety). More recent work has suggested that the sedative and hypnotic effects of these drugs are primarily due to interaction with the cd-containing receptors, therefore much effort has been focused on finding drugs that have preferential activity towards α2β2γ2 and α3βγ2 over α1βγ2 in order to maintain the anxiolytic activity but reduce the sedative side effects (Rudolph, U. F., et al. (1999) Nature 401: 796-800 incorporated herein in its entirety; Low, K. F., et al. (2000) Science 290: 131-134 incorporated herein in its entirety; McKernan, R. M., et al. (2000) Nat. Neurosci. 3: 587-592 incorporated herein in its entirety).
The α5-subunit is predominantly found in the hippocampus, a part of the brain that plays a part in memory and spatial navigation. As a result, much research has been focused on identifying links between α5-containing GABA receptor function and cognition. Results from a number of laboratories have indicated that selective inverse agonism of the α5βγ2/3 GABAA receptor can show marked improvement of memory function in a number of animal models. There have been a growing number of examples of inverse agonists in both the patent and scientific literature (Yokoyama, N., et al. (1982) J. Med. Chem. 25: 337-339 incorporated herein in its entirety; Takada, S., et al. (1988) J. Med. Chem. 31: 1738-1745 incorporated herein in its entirety; Atack, J. R., et al. (2006) European Journal of Pharmacology 548: 77-82 incorporated herein in its entirety). A preferable profile for a cognitive enhancer is one that shows negative modulation at α5, but with less modulation of α1, α2, or α3 to minimize side effects such as convulsion or sedation. As yet, no α5 selective GABAA negative modulator has been brought to market, and only a limited number have been investigated in human clinical trials.